The Exo-S probe class starshade mission The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Seager, Sara et al. “The Exo-S Probe Class Starshade Mission.” Ed. Stuart Shaklan. N.p., 2015. 96050W. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) As Published http://dx.doi.org/10.1117/12.2190378 Publisher SPIE Version Final published version Citable link http://hdl.handle.net/1721.1/106349 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Invited Paper The Exo-S Probe Class Starshade Mission Sara Seager*a, Margaret Turnbullb, William Sparksc, Mark Thomsond, Stuart B Shakland, Aki Robergee, Marc Kuchnere, N. Jeremy Kasdinf, Shawn Domagal-Goldmane, Webster Cashg, Keith Warfieldd, Doug Lismand, Dan Scharfd, David Webbd, Rachel Trabertd, Stefan Martind, Eric Cadyd, Cate Heneghand aMassachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, USA 02139- 4307; bGlobal Science Institute, P.O. Box 252, Antigo, WI, USA 54409; cSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, USA 21218-2410; dJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, USA 91109-8001; eGoddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD, USA 20771-2400; fPrinceton University, Department of Mechanical and Aerospace Engineering, Engineering Quadrangle, Olden Street, Princeton, NJ, USA 08544; gUniversity of Colorado, Center for Astrophysics and Space Astronomy, 389 UCB, Boulder, CO, USA 80309-0389 ABSTRACT Exo-S is a direct imaging space-based mission to discover and characterize exoplanets. With its modest size, Exo-S bridges the gap between census missions like Kepler and a future space-based flagship direct imaging exoplanet mission. With the ability to reach down to Earth-size planets in the habitable zones of nearly two dozen nearby stars, Exo-S is a powerful first step in the search for and identification of Earth-like planets. Compelling science can be returned at the same time as the technological and scientific framework is developed for a larger flagship mission. The Exo-S Science and Technology Definition Team studied two viable starshade-telescope missions for exoplanet direct imaging, targeted to the $1B cost guideline. The first Exo-S mission concept is a starshade and telescope system dedicated to each other for the sole purpose of direct imaging for exoplanets (The “Starshade Dedicated Mission”). The starshade and commercial, 1.1-m diameter telescope co-launch, sharing the same low-cost launch vehicle, conserving cost. The Dedicated mission orbits in a heliocentric, Earth leading, Earth-drift away orbit. The telescope has a conventional instrument package that includes the planet camera, a basic spectrometer, and a guide camera. The second Exo-S mission concept is a starshade that launches separately to rendezvous with an existing on-orbit space telescope (the “Starshade Rendezvous Mission”). The existing telescope adopted for the study is the WFIRST-AFTA (Wide-Field Infrared Survey Telescope Astrophysics Focused Telescope Asset). The WFIRST-AFTA 2.4-m telescope is assumed to have previously launched to a Halo orbit about the Earth-Sun L2 point, away from the gravity gradient of Earth orbit which is unsuitable for formation flying of the starshade and telescope. The impact on WFIRST-AFTA for starshade readiness is minimized; the existing coronagraph instrument performs as the starshade science instrument, while formation guidance is handled by the existing coronagraph focal planes with minimal modification and an added transceiver. Keywords: Exo-S, starshade, external occulter, high contrast imaging, exoplanets 1. INTRODUCTION Thousands of exoplanets and planet candidates are known to exist and the field of planet discovery continues to funnel towards the discovery and identification of an Earth-like planet. While transits—the pioneering and highly successful Kepler [6] and the upcoming TESS mission [7] —are the exoplanet discovery missions of the current generation, space- based direct imaging is required to ultimately find and identify true Earth analogs: Earth-like planets orbiting Sun-like stars. The starshade mission is a space-based, visible wavelength, direct imaging method to search the nearest Sun-like *[email protected]; http://seagerexoplanets.mit.edu Techniques and Instrumentation for Detection of Exoplanets VII, edited by Stuart Shaklan, Proc. of SPIE Vol. 9605 96050W · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2190378 Proc. of SPIE Vol. 9605 96050W-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 01/11/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx stars for planets of all kinds in reflected light, and to characterize both new and already known planets with low- resolution spectra. 1.1 Starshade Conceptual Introduction A starshade (also called an external occulter) is a spacecraft with a carefully shaped screen flown in formation with a telescope (Figure 1). The starshade size and shape, and the starshade-telescope separation, are designed so that the starshade casts a very dark and highly controlled shadow, suppressing the light from the star while leaving the planet’s reflected light unaffected. In this way, only the exoplanet light enters the telescope. Most designs feature a starshade tens of meters in diameter that is separated from the telescope by tens of thousands of kilometers. One might expect, based only on geometric optics, the starshade to be only a bit larger than the diameter of the telescope aperture, circular in shape, and flying in formation close to the telescope. However, diffraction around a circular occulter results in a degraded shadow that is many orders of magnitude brighter than needed for exoplanet imaging. The degraded shadow could be mitigated by employing a much larger and more distant starshade, but the size and distance rapidly becomes prohibitive. Since the early 1960s, it has been known that a circular screen with a radial apodization at large starshade-telescope separations would create a sufficiently dark shadow with a reasonably sized starshade. While such a radial apodization is not manufacturable with sufficient accuracy, it can be approximated using a ring of petals, leading to the special shape of the starshade. Within the family of solutions for the starshade-telescope separation, and the starshade overall size, petal number and shape, the actual solution chosen and its implementation is ultimately driven by engineering design constraints. 30-m or 34-m diameter starshade diameter 1.1 m or 2.4 m Figure 1. Schematic of the starshade-telescope system (not to scale). Starshade viewing geometry with IWA independent of telescope size. Starshade strengths. There are several strengths that a starshade approach brings to exoplanet imaging and characterization. Most significantly, the inner working angle (IWA) and the contrast achieved in the telescope image (the reduction in starlight at the planet location) are mainly a function of the starshade size and distance, not the telescope aperture. A starshade operates by suppressing the light from a parent star before it enters the telescope where it can scatter and hide the very faint planet. Suppression is defined as that fraction of the parent star’s light that is allowed to enter the telescope. Contrast is the amount of background signal in a single telescope resolution element expressed as a fraction of the central star’s brightness. Contrast can be degraded by scattered and diffracted unsuppressed starlight, exozodiacal light, local zodiacal light, and detector dark noise. With a starshade, the starlight is almost entirely suppressed, and the IWA limit at which a planet is visible off the limb of the starshade depends only on the size and distance of the starshade. In principle, even a tiny telescope would be adequate for direct imaging of small exoplanets. In practice, the telescope aperture must be sufficiently large to provide adequate signal and low enough noise from the residual limitations on contrast. Because the starlight never enters the telescope, there is no need for specialized optics to achieve high contrast (which typically reduce throughput), and a relatively simple space telescope is all that is needed. On-axis obstructions or mirror Proc. of SPIE Vol. 9605 96050W-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 01/11/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx segments do not interfere with starlight cancellation and wavefront correction is not required (which frees the telescope from tight thermo-mechanical requirements). An additional significant feature of the starshade-telescope system is the absence of an outer working angle (OWA). A 360° suppressed field of view (FOV) with angles from the star limited only by the detector size is obtained with each image. This is particularly useful for imaging debris disks or planets at large orbital separations, thereby studying planetary systems as a whole. The starshade works over a broad bandpass. Numerically optimized designs balance the desired bandpass with other science drivers and engineering constraints. Hypergaussian designs have no lower limit to their bandpass. The starshade-telescope system can detect Earth-size planets in the habitable zone of Sun-like stars (see Camera: 1K pixels. 21 mas each plus solar system planets 8.44 pc, G05 Beta Canum Venaticorum simulated image of Starshade Rendezvous Mission Figure 2) even with a small telescope (on order of 1- dust ring at 15 AU Hypothetical to 2-m aperture diameter). This ambitious statement is allowed by the fact that nearly all of the starlight suppression is done by the starshade. As long as the tolerances for starshade petal precision manufacturing, Saturn deployment, and formation flying control are met, the Exo-S mission will be capable of reaching the 10-10 contrast level needed to directly observe Earth analog Earth and Venus Exozodi with exoplanets around Sun-like stars.